Electric valve control system



Oct. 19, 19.43. s. R. DURAND 2,331,896

ELECTRIC VALVE CONTROL SYSTEM Filed May 24, 1941 Patented Oct. 19, 1943ELECTRIC VALVE CONTROL SYSTEM Samuel R. Durand, Milwaukee, Wis.,assignor to Allis-Chalmers Manufacturing Company, Milwaukee, Wis, acorporation of Delaware Application May 24, 1941, Serial No. 394,991

20 Ciainis.

This invention relates in general to improvements in electric valvecontrol systems and more particularly to means for controlling theoperation of a group of electric valves of the liquid cathode typeforming part of an electric current converting system and provided witha common evacuating apparatus.

Electric valves of the liquid cathode type utilized in currentconverting systems are generally provided with excitation anodes towhich current is supplied to maintain the cathode in electron emissivecondition regardless of the presence or absence of current through themain anode. When the anodes of a plurality of valves are enclosed withina common casing and are associated With a common cathode, the currentimpulses of the main anodes generally overlap and result in a continuouscurrent which maintains the cathode continuously emissive. It is thenunnecessary to maintain a separate stable excitation arc at the cathodewhile current is flowing through the anodes and such are is needed onlywhen the anodes do not carry any appreciable current.

Valves in which the cathode current is discontinuous, such as the valvesprovided with a single anode, require excitation current under all loadconditions to maintain the cathode emissive between periods of currentconduction of the main anode. It has been found, however, that it isdiificult to maintain, in a valve of this type, either continuousexcitation are or a succession of overlapping periodic arcs under allcontemplated load conditions because of the wide variation of vaporpressure and of Wall temperature rought about by the variations in theload cur- I and the other is stable under high load conditions r of thecathode. One of the excitation arcs is preferably a continuous are whichis sensitive to the gas pressure within the casing. The valves may beconnected to a common evacuating system and the relative gas pressurewithin the different valves or the occurrence of abnormal gas pressurestherein may be determined by observing the variations of theirexcitation currents. These variations may also be utilized forcontrolling the operation of the system.

It is therefore one object of the present invention to provide a controlsystem for an electric valve by means of which the valve may bemaintained conductive with a minimum expenditure of excitation power.

Another object of the present invention is to provide a control systemfor a pluraiity of cooperating electric valves by means of which eachvalve is provided with a plurality of low power excitation arcs whichare stable for different ranges of current intensity through the valves.

Another object of the present invention is to provide a control systemfor a plurality of cooperating electric valves by means of which theflow of current through all the valves may be interrupted uponunsuccessful operation of the ignition apparatus of any valve during apredetermined length of time.

Another object of the present invention is to provide a control systemfor a plurality of cooperating electric valves by means of which thevalves may be rendered non-conductive by disconnecting the excitationanodes of the valves from the source of supply therefor.

Another object of the present invention is to provide a control systemfor a plurality of cooperating electric valves by means of which therelative values of the gas pressures within the different valves may bedetermined from the values of the excitation currents of the valves.

Another object of the present invention is to provide a control systemfor a plurality of cooperating electric valves by means of which theoccurrence of a temporary or permanent excessive increase in the gaspressure of any one of the valves may be determined from the failure ofthe excitation current of the valve,

Objects and advantages other than those above set forth will be apparentfrom the following description when read in connection with theaccompanying drawing, which diagrammatically illustrates one embodimentof the present invention applied to the control of a group of six valvesforming part of an alternating current rectifying system.

Referring more particularly to the drawing by characters of reference, apolyphase alternating current circuit 6 is to be connected witha directcurrent circuit 1 through a convertingsystem comprising a transformer 8and a plurality of interconnected electric valves 9 to M inclusive.Circuit 6 is a supply circuit energized from a suitable generator (notshown) and circuit 1 is an output circuit transmitting current tosuitable load devices (not shown). The load devices may be electricmotors utilized for a service calling for a flow of current of widelyvariable intensity such as electric traction.

Transformer 8 comprises a primary winding l5 subdivided into a pluralityof phase portions connectable with circuit 6 through a switch I1 servingto control the connections between circuits 6 and I through the valves.Winding H5 is inductively related to a secondary winding l8 constitutinga network divided into a plurality of pairs of opposite phase portions.The different pairs of phase portions of winding I8 are severallyconnected with one of the conductors of circuit 1 through pairs ofvalves such as valves 9, I2. is preferably arranged to define aplurality of neutral points severally connected with the other conductorof circuit 1 through an interphase transformer l9.

Valves 9 to M are preferably of a uniform design, each valve comprisinga casing which may be made of insulating or of electrically conductivematerial. The casing 29 of valve 9 and the casings of valves H] to M areconnectedto a common evacuating means such as a pump 2i of the diffusiontype discharging into another pump 22 of the sliding vane rotary type.Pump 2| comprises a resistance heater 23 connected with circuit 6through a transformer 2t. Pump 22 is actuated by a motor 25 connectedwith transformer 24. The connection between pump 2| and the differentvalve casings is preferably effected through a manifold 21 of relativelylarge diameter connected with each casing through an insulating joint asat 28 and through a stop cock or valve as at 29. Valve 9 comprises acathode 39 consisting of a pool of suitable electrically conductivematerial, such as mercury, which is preferably insulated from theassociated casing 20. Valve 9 further comprises a single I. aim anode 3|cooperating with cathode 3G for the flow of current between circuits 6and "I connected therewith.

Each of the valves 9 to 14 is rendered conductive and is maintainedconductive by an ignition-excitation system comprising a plurality ofexcitation anodes. Valve 9 for example comprises an ignition-excitationanode 32 arranged adjacent the surface of cathode 39 at a distance ofapproximately one inch therefrom. Direct current may be supplied toanode 32 from circuit 5 through transformer 24, the'main'contacts of acontrol switch or cont actor 33, a single phase or polyphase transformer34, a suitable rectifying device 35, the coil offa time delay ignitionrelay 36, an electro-responsive device such as an ammeter 32', and aresistor 38. The current of anode 32 returns to rectifier 35 throughcathode 39 and a current smoothing reactor '39.

Momentary engagement of anode 32 with cathode 30 to establish a cathodespot rendering the cathode emissive is controlled by means of a solenoid49 which may be energized from circuit 6 through transformer 26, switch33, and contacts 36a of ignition relay 38. Solenoid 49 may causeengagement of anode 32 with cathode in any known manner, for example, byattracting an armature forming a piston projecting a jet of cathodematerial against'anode 32. An annunciator 4! is connected in parallelwith scle- Winding l 8 v noid 49 to indicate that the ignition system ofvalve 9 has operated. Similar means are severally associated with theother valves for initiating and for maintaining therein continuousexcitation arcs.

Further excitation means provided for valves 9 to M are divided intothree groups each associated with a pair of valves. The two valves of apair, such as valves 9 and I2, are preferably chosen having the mainanodes thereof connected with opposite phases of winding 13. Valve 9 isprovided with a pair of excitation anodes 42, 43, and valve I2 issimilarly provided with a pair of excitation anodes 44, 45. Each ofthese excitation anodes is preferably arranged at a distance ofsubstantially three inches from the associated cathode and is providedwith the usual shielding means for enhancing its stability of operation.

Excitation anodes 42' to 45 receive current from one of the secondaryphase portions 46 of a transformer 41 energized from circuit 6 throughtransformer 24 and switch 39. One terminal of winding 46 is connectedwith excitation anodes 42, 44 through a pair of connections comprisingresistors 48, 49 and these connections are inductively linked by thewinding of a paralleling reactor 59 to force simultaneous flow ofcurrent therethrough. The other terminal of winding 4'5 is similarlyconnected with excitation anodes 43, 45 through another pair ofconnections comprising resistors 5l, 52, and another paralleling reactorThe current of excitation anodes 42 to 45 returns to the midtapconnection provided in winding 46 through a smoothing reactor 54.Similar means are severally associated with the pairs of valves l9, l3and II, M.

Switch 33 controlling the supply of excitation current to valves 9 to 14may be closed by means of a push button switch 5% connecting the switchcoil with transformer 24. Upon closure of switch 33, a holding circuittherefor is completed through the contacts of an overload relay 51 andthe contacts of a time delay lockout relay 58. The coil of relay 5'! isconnected with a current transformer 59 inserted in one of theconductors of circuit 5. The coil of relay 58 may be energized fromrectifier 35 through the contacts of an auxiliary time delay relay 60.The coil of relay 60 may be energized from rectifier 35 through contacts35b of ignition relay 3B or through the similar contacts of any of theother ignition relays, such contacts being connected in parallel. Switch33 is provided with auxiliary contacts 33a for connecting transformer 24with the trip coil Ila of switch I! upon opening of switch 33.

The system being connected as shown on the drawing and circuit 6 beingenergized, pumps 2| and 22 are first operated for a sufficient length oftime to reduce the gas pressure within the valve casings to the desiredvalue as indicated by the usual vacuum meter (not shown). The system maythen be put in operation by momentarily cloesing push button switch 56.Current then flows from transformer 24 through push button switch 56 andthe coil of switch 33. Switch 33 closes and completes its holdingcircuit through the contacts of relays 51 and 58.

Upon closure of switch 33, current is supplied therethroughsimultaneously to the ignition solenoids and to the annunciatorsassociated with all the valves as well as to the coil of relay 60.Solenoid 40 and annunciator'4l, for example, receive current throughcontacts 35a of relay 36, and the annunciator releases its shutter. noid40 attracts the associated piston to cause Sole-' assrees s 3 a jet ofcathode material to impinge on anode 32. An ignition circuit is therebycompleted from rectifier through the coil of relay 36, amrneter 3T,resistor 38, anode 32, cathode 30, and reactor 39 back to the rectifier.Relay 35 immediately opens contacts 35a, thereby disconnecting solenoidfrom circuit 6. The jet of cathode material is thereby broken and theflow of current between anode 32 and cathode 3t] thereupon takes placethrough an are which renders the cathode electron emissive.

Anode 3! does not carry current as switch I! is still open and the arebetween anode 32 and cathode 3B is stable even if only a relatively lowamount of energy is dissipated in its circuit. A stable arc may bemaintainedwhen the output voltage of rectifier 35 is 25 volts andresistor 38 is adjusted to limit the current through anode 32 to twoamperes. The arc is accordingly maintained at anode 32 and maintainsvalve 9 conductive. Continuous excitation arcs are establishedsubstantially simultaneously in all the valves in the same manner. Allthe. excitation relays having operated, relay 69 is deenergized andreturns to the position shown after a predetermined time delay. The coilof relay 58 is thereby deenergized before relay 58 has opened itscontacts.

When the different cathodes are rendered emissive, the potentialsimpressed from transformer 41 on the excitation anodes such as A2, 43initiate additional excitation arcs thereat. More particularly, whenwinding #16 brings anodes 2, 44 to a sufiicient positive potential withrespect to the cathode potential, current impulses flow through anodes42, 44, the fiow of current therethrough being forced to occursimultaneously by the action of reactor 59. This action is repeatedduring every other half cycle of the voltage of circuit 6 to cause theoccurrence of periodic arcs at anodes 42, 44.

During the intervening half cycles, periodic arcs are likewiseestablished at anodes 43, 35 and the flow of current therethrough isforced to take place simultaneously by the action of reactor 53. The twosets of periodic arcs are, however, caused to overlap by the action ofreactor 54 so that anodes 42, 53 for example supply to cathode 38 acontinuous excitation current independently of the current of anode 32.The current of reactor 54 may be of intensity as low as eight ampereswhen the voltage of winding has a value of thirty volts.

When anode 31 is without current, the arcs of anodes 42, 53 would not besufiiciently stable in I the absence of current at anode 32, but the arcof anode 32 is stable under such conditions, and maintains the arcs ofanodes 32, G3 in stable operation. The excitation arcs being establishedin the valves, switch I! may be closed. Valves 9 to it then transmitcurrent from circuit 5 to circuit 7 in the well known usual manner.

The flow of current through each valve takes place in non-consecutiveperiodic impulses. For example, anode 3i carries a current impulse onceduring every cycle of the voltage of circuit 8 when the anode is broughtby Winding E8 to a potential which is higher than the potentials of theother anodes. Anode 61 of valves l2 carries current impulses which occurone-half cycle later than the current impulses of anode (ii. The currentimpulses of the different valves combine into a substantially uniformflow of current delivered to circuit 7. After closure of switch ll theannunciators are all reset by any preferred known means in order thatthey may be able to indicate subsequent operations of the ignitionsystem. 7

When the flow of current through circuit 7 is of relatively lowintensity, below one hundred and fifty amperes, the continuousexcitation are attaching at anode 32 is stable and maintains the arcs ofanodes 42 and 43 in stable operation. When the flow of current throughthe circuit exceeds one hundred and fifty amperes, the vaporized cathodematerial within casing 25 is at relatively high pressure and at arelatively high temperature. The vapor also reaches a high degree ofionization during the periods of current flow through anode 31. Underthese conditions the relatively short are of anode 32 would not besufficiently stable in the absence of relatively long arcs at anodes 42,43.

The latter arcs are inherently stable during the portion of the voltagecycle in which the voltages impressed on anodes 42, 4-3 are relativelyhigh but they are not reliably stable. when the voltages impressed onthe anodes are relatively low. The phase relation of transformers 8 andd? is so chosen that during each cycle the voltage of winding itreverses once during the current conducting period of anode 3| and onceduring the current conducting period of anode 6 i.

Let it be considered that at a particular moment anode 3! is carryingcurrent, while anodes t2, 34 also carry current under a decreasingvoltage. When the potential of anodes 32, at becomes lower than thepotential of anodes t5, the flow of current through anodes 12, id istrans ferred to anodes t3, 55 by initiation or" arcs thereat. The highdegree of ionization of the vapor in valve 9 resulting from the flow ofa high current through anode 3| causes the establishment of the arc atanode 43 to take place instantly without difficulty. The conditions areless favorable in valve l2 in which ionization is then produced only bythe are at the ignition excitation anode. If the arc is not immediatelyestablished at anode 25, the flow of current through anode 3 and throughreactor 53 causes impression on anode 45 of an inductive voltage impulsewhich is sufficient to cause formation of an arc atancde 45 to take overthe current of anode id.

One-half cycle later current is to be transferred back from anodes 43,45 to anodes 22, dd. Anode (ii is then carrying current and the tran ferof current from anode (=5 back to anode it takes place withoutdifiiculty. If the transfer of current from anode d3 back to anode 52fails to take place immediately, the flow of current to anode M throughreactor 59 causes impression on anode 512 of an inductive voltageimpulse which causes anode 52 to take over the current of anode 63. Thearc at anode 42 or anode it is thus maintained stable at every instant,thereby insuring the stability of the are at anode Stable excitationarcs are likewise maintained in the other valves by the combination ofexcitation anodes and of paralleling reactors above described.

The ignition resistors are preferably adjusted so that the readings ofthe ammeters are all equal. While the arc at anode 32 is stabilizedabove set forth under all conditions of vapor pressure and temperatureresulting from variation of the current of anode 3i, this stability isaffected by the presence of foreign gases within casing Eli. These gasesmay be air leaking through a defective seal of the casing or othergasesreleased from the material utilized in the valve as a result ofthe. operation thereof-Q:

If gases appear in casing 20 for example, at a faster rate. than theycan be removed by pumps 21, 22, the gas pressure within the casingexceeds the valuenormally maintained therein by operation, of thepumpsand-the current indicated by ammeter 31 decreases to an extentcommensurate with the increase in gas pressure. The gases,

however, do not generally diffusefrom casing'il into the casings of theother valves because the connection between. the casings is effectedthrough manifold 21, which is constantly being evacuated by pumps. 2|,22. Inspection of the ammeters will therefore showthat the excitationcurrent of anode32 is lower than the currents of the correspondinganodesof the other valves, thereby indicating that thegas pressure invalve 9 is relatively greater than the gas pressure within the othervalves.

If the gas pressure within casing 23 becomes excessive, thearc at anode32 becomes unstable and subject to extinction even though thearcs atanodes 42, 43 and? may not be so affected Upon failure of the flow ofcurrent through valve 9 by Way of anode 32, the ignition system associated with valve 9 is caused to operate.- More particularly, relay 36returns to -the position shown to reconnect solenoid 40 with transformer24. The solenoid again attracts the-armature thereof to reestablish theare at anode 32. If the increase in gas pressure in casing-'28 was onlymomentary, the arcmay be reestablished after a single operation of theignition system or after a small number of such operations.

If the gas pressure remains excessive during a period of severalseconds, relay 35- continues to operate to attempt to reestablish thearc. After each operation relay 36 returns to the position shown,thereby connecting the coil of relay G8 with rectifier 35. Upon thefirst return of relay 36. to the position shown, relay E6instantaneously attracts its armature. Relay 5B is provided withvsuitable time delay means whichv If the operation of relay E6 continuesfor a predetermined length of time depending on the adjustment of relay58, the latter relay opens its contacts to deenergize the coilof switch33.

Switch 33 opens, thereby rendering the ignition means of all the valvesineffective and simulta- .eously disconnecting thevexcitation anodes ofall the valves from circuit 6. The flow of current through the valves'isthereby interrupted, the valves being rendered non-conductive for lackof excitation current at the cathodes thereof. When switch 3 has.returned to the position shown, contacts 33a. connect trip coil lid ofswitch ll with transformer 24. The trip coil withdraws latch llb,thereby causing switch 1? to open under the action of spring 110;

The operation of relay 35 also causes operation of annunciator 4| whichreleases its shutter to indicate that relay 36- has operated in responseto failure of the flow of current through anode 32. The otherannunciators, however, have not operated, so that anindication isobtained that the gas pressure was-excessive only in valve 9. may thenbe disconnected from manifold. 21; and the'rate of increase of'the gaspressure inucasf- Valves ID to .M- andpumpsll, 22'

Ill)

ing. 26 and manifold Z'Lmaybe determined by means of the commonvacuummeterlnot shown) usually. associated with the: evacuating system.If-valve 91has become permanently defective, stop cook 29 is closed andvalve: 9 Imay be removed after disassembling its-different electricalconnections and joint 28; The valvemaythen be repaired andreinstalled'or a spare valve may be substituted therefor.

If the flow of current through: valve 9 becomes excessive because ofoverloading of circuit 1 or because of a backfire in any one of thevalves, current transformer 59 supplies tothe coil of relay 51 a currentof sufficient magnitude tocause the relay to operate. Relay 5'! .opensthe circuit of the coil of switch 33, which causes interruption: of theflow of current through the valves and opening of switch I! in themanner above described. Thisoperation, however, does not cause any oftheignition relays to function and none of the annunciators operates,thereby indicating that the disconnection of the system was not due toexcessive gas pressure within one of the valves.

Although but oneembodiment of the invention has been illustrated anddescribed, it will be apparent to those skilled in the art that variouschanges and modifications may be made therein without departing-from thespirit of the invention or from the scope of the appended claims. someofthefeatures ofthe invention disclosed and claimed herein are alsodisclosed in Patent No. 2,251,484, issued August 5, 1941, to Samuel R.Durand, upon an application Serial No. 297,- 210, filed September '30,1939;

It is claimed and desired to secure by Letters Patent:-

1. The combination of a supply circuit, an output circuit, a pluralityof electric valves each comprising an anode and a cathodeconnected withsaid circuits for the flow of energy therebetween, means for controllingsaid flow of energy comprising a plurality of. means each including anelement of one of said .valves for rendering said'valves' conductive,and means for interrupting the flow of current between said circuits byway of said valves comprising means responsive to continual operation ofthe first said means during a predetermined 'length'of time.

2. The combination of a supply circuit, an output circuit,- a pluralityof electric valves each comprising an anode and-a cathode connected withsaid circuits for the flow of energy therebetween, ignitionmeansseverally associated with the different said valves for renderingsaid valves conductive, means responsive to failure of the flow ofcurrent through any one of saidvalves for causing operation of the saidignition means associated with the said one of said valves, and meansfor, interrupting the flow of current between said. circuits by Way ofsaid valves comprising, means responsive .to continual. operation of anyone of said ignition meansduring a predetermined length of time.

3. The combination of a supply circuit, an output circuit, a pluralityofv electric valves each comprising an anode anda cathode connected withsaid circuits for the flow of energy therebetween, a switch forcontrolling the connection between said circuits through said valves,means for-controlling said fiowof energy comprising a plurality ofmean-each including an. element of one of said valves for rendering saidcathodes emissive, and means responsive totan operating condition ofsaid valves for rendering the first said means inefiective and forcausing opening of said switch.

4. The combination of a supply circuit, an output circuit, a pluralityof electric valves each comprising an anode and a cathode connected withsaid circuits for the flow of energy therebetween, a switch forcontrolling the connection between said circuits through said valves,ignition means severally associated with the difierent said valves forrendering said cathodes emissive, means responsive to failure of theflow of current through any one of said valves for causing operation ofthe said ignition means associated with the-said one of said valves, andmeans responsive to continual operation of any of said ignition meansduring a predetermined length of time for rendering said ignition meansineffective and for causing opening of said switch.

5. The combination of an electric valve comprising a cathode andexcitation anode means, means for rendering said valve conductive, meanscomprising a source of current connected with said excitation anodemeans for supplying a continuous excitation current to said cathode tomaintain said valve continuously conductive, and means for renderingsaid Valve non-conductive comprising mean responsive to an operatingcondition of said valve for disconnecting said source from saidexcitation anode means.

6. The combination of a supply circuit, an out put circuit, a pluralityof electric valves each comprising an anode and a cathode connected withsaid circuits for the flow of energy therebetween and excitation anodemeans, means for rendering said Valves conductive, means comprising asource of current connected with said excitation anode means forsupplying continuous excitation current to each of said cathodes tomaintain said valves continuously conductive, and means for renderingsaid valves non-conductive comprising means responsive to an operatingcondition of said valves for simultaneously disconnecting all saidexcitation anode means from said source.

7. In a control circuit for an electric valve comprising a main anodeand a cathode for the flow of current of variable intensity, a pluralityof excitation anodes arranged within said valve, means comprising asource of alternating current connected with said cathode and with saidexcitation anodes for producing a plurality of overlapping periodicexcitation arcs therebetween which are stable during fiow of current ofrelatively high intensity between said main anode and said cathode, andmeans comprising a source of direct current connected with said cathodeand with one of said excitation anodes for maintaining a continuousexcitation arc therehetween which is stable during flow of current ofrelatively low intensity between said main anode and said cathode.

8. In a control circuit for an electric valve comprising a main anodeand a cathode for the flow of current of variable intensity, a pluralityof excitation anodes arranged within said valve, means comprising asource of direct current connected with said cathode and with one ofsaid excitation anodes and means for causing momentary engagement ofsaid cathode with said one of said excitation anodes to initiate andmaintain therebetween a continuous excitation are which is stable duringflow of current of relatively low intensity between said main anode andsaid cathode, and means comprising a source of sity therebetween, meanscomprising a source of direct current connected with said cathode andwith one of said excitation anodes for producing a continuous excitationare which is stable durthe intervals between said current impulses andduring the flow of said current impulses of relatively low intensity,and means comprising a source of alternating current connected with saidcathode and with others of said excitation anodes for producing aplurality of overlapping periodic excitation arcs therbetween which arestable during fiowoi said current impulses of relatively high intensity.

10. The combination of a plurality of interconnected electric valveseach comprising a single main anode, a cathode, and aplurality ofexcitation anodes, means comprising a source of periodic currentconnected with said main anodes and with said cathodes to cause the flowof periodic current impulses alternately between the difierent said mainanodes and said cathodes, and means for maintaining excitation arcs insaid valves comprising a source of alternating current connected withsaid cathodes, a plurality of connections between said source ofalternating current and a group of said excitation anodes associatedwith different ones of said cathodes for the supply of current impulsesfrom said source of alternating current to said group of excitationanodes, and means inductively linking said connections for causing theperiods of occurrence of said current impulses to coincide.

11. The combination of a pair of interconnected electric valves eachcomprising a single main anode, a cathode, and excitation meanscomprising a pair of excitation anodes, means comprising a source ofperiodic current connected with said main anodes and with said cathodesto cause the flow of periodic current impulses alternately between thesaid main anodes and the said cathodes, and means for maintainingexcitation arcs in said valves comprising a source of alternatingcurrent connected with said cathodes, a pair of connections between saidsource of alternating current and one excitation anode of each or saidvalves to cause simultaneous impression thereon of arc initiatingpotentials during flow of current through one of said main anodes, meansinductively linking said connections to force simultaneous flow ofcurrent therethrough, a second pair of connections between said sourceof alternating current and the reiaining said excitation anodes to causesimultaneous impression thereon of arc initiating potentials during flowof current through the other one of said main anodes, and meansinductively linking the second said connections to force simultaneousflow of current therethrough.

12. The combination of a pair of interconnected electric valves eachcomprising a single main anode, a cathode, and a plurality of excitationanodes, means comprising a source of periodic current connected withsaid main anodes and with said cathodes to-cause the flow -of pcriodiccurrent impulses alternately between-the said main anodes and the saidcathodes, and means-for maintaining excitation arcs in said valves"under all conditions of current flow therethrough comrising a sourcevofalternating current connected with said cathodespa pair of connectionsbetween said source of alternating current and one excitation anode ofeach of said valves to cause simultaneous impression thereon of arcinitiating potential during flow of current through one of said mainanodes, means inductively linking said connections to force simultaneousflow of current therethrough, a second pair of connections between saidsource of alternating :current and another excitation anode of each ofsaid valves to cause simultaneous impression thereon of arc initiatingpotential during flow of current through the other one of said mainanodes, means inductively linking the second said connections to forcesimultaneous flow of current therethrough, and a source of directcurrent connected with said cathodes and with-a third excitation anodeof each of said valves to maintain continuous excitation arcstherebetween.

13. The combintaion of a plurality'of interconnected electric valveseach comprising a casing, main anode means and a cathode, means formaintaining said cathodes continuously emis'sive comprising anexcitation anode ineach of said casings and a source ofdirect'current'connected with said excitation anodes and withsaidcathodes, evacuating means connected with each of said casings, andmeans for indicating the relative values of the gas pressures within thedifferent said casings comprising a plurality of electroresponsivedevices severally connected between said source and the different saidexcitation anodes.

14. The "combination of a plurality of interconnected electric valveseach comprising a cas ing, main anode means and a'cathode, means formaintaining said cathodes continuously emissive comprising an excitationanode in each of said casings and a source of direct current connectedwith said excitation anodes and with said cathodes to maintainexcitation arcs therebetween, each said excitation are being subject toextinction upon excessive increase of the gas pressure within thecorresponding casing, ignition means severally associated with thedifierent said valves for establishing said excitation arcs, relay meansresponsive to failure of the said excitation arc of any one of saidvalves for causing operation of the said ignition means associated withthe said one of said valves, evacuating means connected with each ofsaid casings, and means for indicating in which of said casings anexcessive gas pressure occurs comprising indicating means responsive tooperation of said relay means.

15. The combination of a supply circuit, an output circuit, a pluralityof electric valves each comprising a casing containing main anode meansand a cathode connected with said circuits for the flow of energytherebetween, a switch for controlling the connection between saidcircuits through said valves, means for maintaining said cathodescontinuously emissive comprising an excitation anode in each of saidcasings and a source of direct current connected with said excitationanodes and with said cathodes to maintain excitation arcs therebetween,each said excitation are being subject to extinction upon excessiveincrease of the gas pressure within the corresponding casing, ignitionmeansseverally associated with the --different said valves forestablishing said excitation arcs, relay means responsive to failure ofthesaid-excitation arc of anyone of said valves-for causingoperation ofthe said ignition means associated with "the said one of said valves,means responsive to operation of said relay means duringa-predetermined' length of time for rendering said relay-meansinefiectiveand for causing opening of said switch, evacuatin meansconnected with each of said casings,and means for indicating in which ofsaid casings an excessivegas pressure resulted in the oper ationof saidswitch comprising indicating means responsive to operation of said relaymeans.

16. The combination with a supply circuit, an output circuit,a pluralityof electric valves each having ananode and a cathode connected with saidcircuits for the flow of current therebetween and each having-excitationanode means/ignition means distinctirom said excitation anode meansseverally associated withthe different said cathodes for rendering saidvalves conductive, and meanscomprising a source of current connectedwith said excitation anode means for supplying continuous excitationcurrent to each of said cathodes to maintain said valves continuouslyconductive independently of said ignition means, of means for renderingsaid valves simultaneously non-conductive for the flow of currentbetween said circuits comprising timing means responsive to initiationof the operation of said ignition means for rendering said excitationcurrent supply means inoperative after a predetermined time interval.

17. The combination with a supply circuit, an output circuit, aplurality of electric valves each having an anode and a cathodeconnected with said circuits for the flow of current therebetween andeach having excitation anode means, a switch for controlling theconnection between said circuits through said valves, and ignition meansfor rendering and maintaining said valves conductive comprising a sourceof current connected with said excitation anode means forsupplyingexcitation current to each of said cathodes, of means responsive to anoperating condition of said valves for rendering said ignition meansinoperative and thereafter causing opening of said switch.

18. The combination with a supply circuit, an output circuit, aplurality of electric valves each having an anode and a cathodeconnected with said circuits for the flow of current therebetween andeach having excitation anode means, a switch for controlling theconnection between said circuits through said valves, and ignition meansfor rendering and maintaining said valves conductive comprising a sourceof current connected with said excitation anode means for supplyingexcitation current to each of said cathodes, of timing means responsiveto the operation of said ignition means for rendering said ignitionmeans inoperative and thereafter causing opening of said switch after apredetermined time interval.

19. The combination with a supply circuit, an output circuit, aplurality of electric valves each having an anode and a cathodeconnected with said circuits for the flow of current therebetween andeach having excitation anode means, ignition means distinct from saidexcitation anode means severally associated with each of said valves forrendering said valves conductive, and excitation means comprising asource of current connected with said excitation anode means forsupplying current to each of said cathodes to maintain said valvescontinuously conductive independently of said ignition means, of relaymeans responsive to failure of the flow of excitation current throughany one of said valves for causing operation of the ignition meansassociated with said one of said valves, and timing means responsive tocon-- tinued operation of said relay means during a predetermined timeinterval for rendering the ignition and excitation means of all saidvalves inoperative.

20. The combination with a supply circuit, an output circuit, aplurality of electric valves each having an anode and a cathodeconnected with said circuits for the flow of current therebetween andeach having excitation anode means, a switch for controlling theconnection between said circuits through said valves, ignition meansseverally associated with each of said Valves for rendering said valvesconductive, and excitation means comprising asource of current connectedwith said excitation anode means for supplying current to each of saidcathodes to maintain said valves continuously conductive, of relay meansresponsive to failure of the flow of excitation current through any oneof said valves for causing operation of the ignition means associatedwith said one of said valves and timing means responsive to continuedoperation of said relay means during a predetermined time interval forrendering the ignition and excitation means of all said valvesinoperative and for thereafter causing opening of said switch.

SAMUEL R. DURAND.

